In various types of chemical analysis there often arises a need for an accurate metering type pump which is capable of delivering a discrete quantity of one or more fluid samples to a container where the samples are mixed and analyzed. A water pollution-control system is an example of where such a pump is needed. In such a system a sample of the water being tested is mixed with predetermined quantities of various chemical constituents. Often it is necessary to automatically monitor the pollution level of the water in the system, and for this type of application a pump is needed which is capable of withdrawing a precisely measured quantity of water sample and mixing it with equally precise quantities of other chemical constituents needed for the analysis.
Pulsation type metering pumps have been used in applications of the type described where precision is important. In construction, these pumps usually include a pump body equipped with an inlet-outlet means and a flexible resilient walled tubular conduit connected between the inlet-outlet means. With such constructions, discrete precisely measured quantities of fluid are moved through the pump by squeezing the conduit section located between the inlet and outlet means. Valve means are usually provided in the pump to control the direction of fluid flow.
Generally, the squeezing action causes flexure of the conduit in a direction transverse to the length of the conduit. This flexure, in turn, causes compression of the conduit and thus a reduction in the internal volume thereof. In response to this volume reduction, a discrete quantity of fluid encased within the conduit is moved through the pump. The amount of fluid flow is, of course, equal to the amount of volume reduction which occurs as a result of the transverse flexure of the conduit wall. And, fluid flow with such pumps substantially ceases upon removal of the squeezing pressure.
One construction of the type referred to above is described in my co-pending U.S. Pat. application Ser. No. 31,686, filed Apr. 24, 1970 for Metering Pump and Valve, now U.S. Pat. No. 3679331.
Although prior art pulsation-type pumps provide several advantages, especially where precision is required, there are also certain drawbacks associated with these pumps. First of all, there is a tendency for solid particles and other impurities contained within the sample fluid to become entrapped within the conduit section between the inlet and outlet means of the pump. When this occurs the pump cannot operate with the required degree of precision. And, if enough particulate material becomes entrapped the conduit may become completely clogged causing stagnation of the fluid encased within the conduit. This condition is undesirable generally but it is especially undesirable when the pump is used in an application requiring continuous monitoring, as for example, in a pollution-control system.
In addition to particulate matter, air bubbles sometimes collect and become entrapped within the conduit. This phenomenon has the same undesirable effect on pump operation as instances of severe clogging attributable to entrainment of particulate material.